Hi vee, I only have a small amount of info in your post to go on and i'm sure tiktok or someone else will be along soon, however to break down your post this would be my conclusions, "cross and lifting legs" is normally wind / tummy pain. constant feeding in the evening is usually referred to as cluster feeding and is very normal, your milk supply is not considered fully established at 3 weeks and by cluster feeding in the evenings your baby is helping to set up your supply and fill up for a few hours. Green poo is generally considered to be a sign of too much foremilk (and can make babies windy) so it may be worth keeping her on each side for longer, it can also help to take your baby off and wind after the 1st letdown and then put back on the same side. ( lots of babies gulp when the let down is fast and take in air)
That failing get your latch checked, at this age its all latch , latch , latch. If your latch isn't good now it can cause supply problems later, so its worth checking.
Personally I think that for a long time it wasn't realised that some babies were reacting to dairy, however it now seems to be the 1st assumption and that isn't ideal either. My dd1 is v sensitive to dairy, not an allergy in the usual sense as she becomes severely congested to the point of being deaf as opposed to being sick.
I am not a qualified expert, but I have b/fed all of my 4 children, my youngest is 15 weeks and I have now been feeding for fifty five months.
You sound as tho you are being very sensible in your approach to the problem and well done on making it this far
By the way, who is they??? HV / dr / midwife / lactation consultant?